Questions(maybe silly) about Transformers

[zenocosini]

Hi all,
maybe the questions are stupid but I can’t find the answer by myself.
Waiting for 4780 kit (Gainclone) I’ve bought a 300VA transformer with 15(!) wires :
pri.: 0-220-240V brown-blue-red
sec.1 2x 0-17-21V black-green-white
sec.2 2x 0-17-21V black-green-white

if I’ll connect only 2 secondaries (one for each channel) the transformer will provide only 150VA ?
Is it best to connect one channel of the ampli to sec1 and the other to sec2 or should I connect both channel to sec1 (or sec2)?

Thanks in advance.

 

[wes-ninja250]

Wow, that's quite the transformer.

If you are using one power supply board, you will probably need to parallel two sets of secondaries, and also run them in series if you are only running one bridge recitifier.

Personally, I would build two BrianGT-style power supply boards (chipamp.com), and run one secondary to each AC in (four secondary windings in all).

Wes

 

[wes-ninja250]

If I understand both you and PSUs correctly, this attachment represents all three reasonable ways that I can think of to hook up the transformer you've selected. Of course, it is possible that I misunderstand one or both.

I would use the top left if it were my choice, and would avoid the bottom one with great vigour.

Finally, all but the first one assume that it is safe to connect the secondaries on your transformer in parallel. This is generally true in the case of toroidal transformers, but you should consult the datasheet to be safe.

Wes

 

[zenocosini]

Thanks for answering Wes,
I have no data sheet.
Since I don't feel confident in connecting in parallel I was wandering if there is something wrong in connecting only two secondaries like this:

 

[wes-ninja250]

The problem is that you are not using half your transformer, so will have only 150VA at your disposal.

I would strongly suggest consulting the transformer's data sheet for parallel installation information --- or simply build an extra power supply board as in the top-left diagram in my attachement.

Wes

 

[zenocosini]

Ok, everything clear!
Many thanks Wes, I resolved to build the extra power supply as you suggest .
Anything wrong If I'll use a pair of "ready made" bridge like KBPC04-25 togeter the two supplied with the kit that are standalone diodes?

 

[wes-ninja250]

I would strongly suggest matching the power supplies, otherwise you will almost certainly have one channel that sounds different from the other.

Wes

 

[AndrewT]

Hi,
is this transformer for a stereo amplifier?
Then wire one pair of windings through one rectifier into a series pair of smoothing caps. Wire the other pair of secondaries through another rectifier and series pair of smoothing caps.
Connect the spare 0Vac and 21Vac wires together to form the centre tap and connect this to the capacitor common for that same winding forming your ground connection. Form the other cap common and centre tap to create your second ground connection. Run both these ground connections to your central star ground.
You now have +-dc twice for your stereo amplifiers.

 

[zenocosini]

I'm not sure to have completely understood your configuration AndrewT.
Anyway much ado about nothing! sorry :-(

I've already realized that the kit I've bought http://www.audiosector.com/lm4780.shtml) is the dual mono version so the transformer should suit it perfectly in the configuration that Wes suggested (the first left in his attachment).
It seems that besides you, Wes and Andrew, even luck tries to help me

Thank you